In 2002 my elder son Henry was diagnosed with schizophrenia. His final breakdown came when he tried to swim Newhaven estuary in mid-winter fully clothed and was lucky to be rescued by some fishermen before he died of cold. He spent the next eight years in and out of mental hospitals, though more recently has shown strong signs of recovery.

Henry later said that between the ages of 14 and 19 he had taken marijuana almost every day. He did so because it made him feel less shy with other people and his generation of teenagers all did so too. The music-scene, of which he wanted to be part, was drug orientated. “It would have been better if I hadn’t,” he says now, “but about half the people I knew in Canterbury [where he lived] were smoking dope.”

At the time I knew little about cannabis and nothing about mental illness. Henry was very intelligent, charming and passed all his exams. I was at Oxford in the late 1960s and early 1970s when cannabis smoking was becoming popular, but I had probably puffed at a joint about twice in my life. I never knew about the dangers involved.

The risks are quite specific in the case of cannabis and tend to be masked by blanket disapproval or disapproval of the drug. Not everybody taking it is vulnerable to the same degree, but numerous scientific studies show that cannabis can be the precipitating factor for the sizeable minority of the population, perhaps 20 per cent, with a genetic pre-disposition to psychosis.

The evidence for this has been mounting for decades. A study of 1,900 people between the ages of 14-24 published in the British Medical Journal this year, who at the start of the survey had not taken cannabis, were reassessed at three and eight year periods. Those who took to smoking cannabis had a higher risk of developing psychosis. Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said “this study adds a further brick to the wall of evidence showing that use of traditional cannabis is a contributory cause of psychoses like schizophrenia.”

The conclusion mirrors my own experience. When Henry first showed symptoms of mental disorder I was amazed to discover how many friends, many of whom I had known well for years, had a close relative disabled by schizophrenia whom they had never told me of. Again and again the common feature in these tragedies was that the victim had taken cannabis in significant quantities at a young age.

It is this failure to take adequately on board the dangers of cannabis which is a serious failing of an otherwise impressive report by the Global Commission of Drug Policy published last week which is signed by luminaries such as the former UN Secretary General Kofi Annan and former chairman of the Federal Reserve Paul Volcker. All agree that the 40-year-old “war on drugs” has been a failure at every level and suggest other approaches such as decriminalization, education and treatment to replace incarceration.

The report was predictably rejected, despite its high level endorsement, by politicians across the world who learn from the cradle that a policy of “being tough on drugs” however counter-effective is invariably popular. But there is a second, less mindless reason why a well-researched report like this has less impact than it should. This has because supporters of decriminalization in the media and among the intelligentsia in general see cannabis as harmless and discount opposition to it as ill-informed prejudice.

Even in a well-informed report like this there are repeated signs of an underestimation of the dangers of some drugs. For instance it blithely reproduces a chart published in Lancet drawn by “independent experts” claiming that cannabis carries a lower risk to the consumer than alcohol and cigarettes. They fail to make clear that for people genetically susceptible to psychosis the risks involved in taking cannabis may be lethally high. Cigarettes and alcohol, whatever harm they cause, do not, at a young age, send you mad. The report would carry more political punch if it underlined that its proposed “experiment in decriminalization” does not imply that cannabis is less harmful than was once believed. It may, indeed, be much more dangerous.

It is not as if there were only a small number of people at risk. An estimated 51 million people worldwide, including 2.2 million in the US and 250,000 in the UK, are estimated to suffer from schizophrenia, which may have been precipitated or hastened as to the time it manifests itself by cannabis, so the potential casualty list from ill-judged experimentation is large.

The proponents of current drug policies, largely devised and propagated by the US, scarcely bother to justify them in rational terms. The present demagogic approach is usually attributed to President Nixon, though it was first used as a political weapon by his Republican rival the New York governor Nelson Rockefeller. As with “red-baiting” in the US in the 1940s and 1950s and “the war on terror” post 9/11, the “war on drugs” was always a means of manipulating and exaggerating public fears as a means of winning political power and as Nixon put it privately to his aide Haldeman, essentially aimed at America’s blacks.

The results, as the Global Security’s report cogently explains, have been devastating. Demand and profits will always be so high that arrests of dealers, punishment of consumers, crop eradication, highly publicized seizures of drug shipments, have no overall effect. Where an obstacle to the drug trade appears, the trade simply flows through different people and countries. States with weak security systems like Mexico and the Central American states or, increasingly, West Africa, do not have the strength to combat the hordes of gunmen which drug money can pay for.

Security agencies justify their bloated budgets as participants in the “war on drugs” despite its manifest failure. Cheaper and demonstrably more effective methods are under-funded or neglected. For instance, provision of clean syringes to heroin users in UK and Germany has kept HIV prevalence among people who inject drugs to below five per cent while in Russia and Thailand, where there is no such provision, it is over 35 per cent.

The war on drugs incorporates and exacerbates racial and social conflict. Of the 2.3 million people in jail in the US about a million are there for non-violent offences, mostly to do with drugs, and of these half are African-Americans and a quarter Latinos. Despite ruining the lives of so many non-criminal consumers, imprisonment has not reduced drug consumption or dented the profits to be made.

None of this is going to change while the US plays the predominant role in determining the drugs policies of the rest of the world. The “war on drugs” is too politically attractive in the US to be abandoned as futile. The report rather daringly denounces this as ‘drug control imperialism’ that lands the rest of the world with a failed American policy while more successful drug policies tested successfully in European countries are ignored.